Sputtering as a viable route for In2S3 buffer layer deposition in high efficiency Cu(In,Ga)Se2 solar cells

Alternative buffer layers in CIGSe are deposited mainly using chemical bath deposition because of its benefits like simplicity, good film quality and surface/step coverage. All the layers in CIGSe cell stack such as back contact, absorber and window layers are deposited by vacuum-deposition methods such as coevaporation, sputtering, and sometimes thermal evaporation, except for the buffer layer. Therefore, in the present work we demonstrate the feasibility to deposit In2S3 by RF magnetron sputtering reaching cell efficiencies of 13.6%, which is the highest value available for sputtered In2S3 in literature to date. Absorber surface damage and nonuniform buffer layer thickness are the primary limitations when using sputtering, and hence need to be eliminated for reaching reasonable cell efficiencies. We studied the extent of sputter induced damage on CIGSe absorber as well as the sputtering- and annealing-induced intermixing phenomenon at the In2S3/Cu(In,Ga)Se-2 interface at the subnanometer level using atom probe tomography. We have also shown that a post deposition annealing not only significantly improves the crystallinity of In2S3, but also recovers the surface damage caused by sputter-induced intermixing resulting in an improved p-n Junction quality (as shown by the electron beam induced current investigations), and substantially improves cell efficiency. The present work opens a new way for designing efficient and industry-compatible CIGSe cells using sputter-deposited Cd-free buffer layers. Moreover, this work clearly demonstrates that this novel and fully vacuum-deposited CIGSe cell meets the standard requirements, in terms of chemistry, structure, and electrical performance of a working cell for the PV industry.